The answer is pretty easy: Your .center class IS centred. But your price-tables have a width of 25% - that means as long as you have only three container with that class there will be a space with the width of 25% on the right hand side.
Solution: Eather add a fourth price-option or set the width of .cp-column to 33% ;)
Now it looks like:
.center {
margin-left: auto;
margin-right: auto;
}
.cp-column {
width: 25%;
float: left;
}
Change that to
.center {
margin-left: auto;
margin-right: auto;
}
.cp-column {
width: 33%;
float: left;
}